U.P.B. Sci. Bull., Series C, Vol. 72, Iss. 1, 2010
ISSN 1454-234x
INFLUENCE FACTORS ON THE TRANSMITTED
OVERVOLTAGES FROM HIGH VOLTAGE TO LOW
VOLTAGE NETWORKS
Marian COSTEA1, Bogdan NICOARĂ2
În reţelele electrice de înaltă şi medie tensiune poate apărea o gamă largă de
supratensiuni. Acestea pot fi iniţiate de trăsnete, de manevre efectuate cu
echipamente de comutaţie sau de cedări de izolaţie. Protecţia împotriva
supratensiunilor, realizată cu descărcătoare sau bazată pe acţiunea releelor de
protecţie are ca scop eliminarea sau reducerea solicitărilor dielectrice ale
izolaţiilor. Cu toate acestea, supratensiunile tranzitorii care apar în reţelele de
tensiune înaltă sau medie se transmit, prin diferite mecanisme, reţelelor de joasă
tensiune. Lucrarea analizează factorii de influenţă şi ponderea acestora asupra
supratensiunilor transmise de la o reţea de tensiune superioară către o alta, de
tensiune inferioară, luând în considerare funcţionarea adecvată a descărcătoarelor
din reţeaua de tensiune superioară. Studiul a fost realizat pe o configuraţie existentă
de reţea, supusă unor supratensiuni de trăsnet având diverse forme şi amplitudini.
In the high and medium voltage networks a large variety of overvoltages can
occur. These are produced by lightnings, switching operations or insulation faults.
Normally, the protection against overvoltages, based of surge arresters and relay
protection of the equipment, act to eliminate or reduce the insulation stress. The
transients occurred in the high or medium voltage networks are then transmitted by
different mechanisms to the low voltage networks. The paper analyse the influence
factors and their weights to the transmitted overvoltages from a superior voltage
networks to the low voltage one considering a proper action of the protection
against overvoltages in the first network. The study was performed on a real
configuration of network subjected of different shape and amplitude of lightning
overvoltages.
Keywords: electric power installation, full lightning impulse, chopped lightning
impulse, lightning induced overvoltage
1. Introduction
Transient events which occur in the high or medium voltage installations
produced by switching operations or lightning are transmitted to the final low
voltage user by propagation along the transmission or distribution lines and
through inductive and capacitive couplings between the windings of transformers.
1
2
Assoc. prof., Power Engineering Faculty, University POLITEHNICA of Bucharest, Romania
Prof., Power Engineering Faculty, University POLITEHNICA of Bucharest, Romania
184
Marian Costea, Bogdan Nicoară
The parameters of lines and transformers have a major influence to the
transmitted overvoltages. But the behaviour of the lines, transformers, arresters
and other equipment of the network depend of the frequency and voltage level
and, consequently, of the shape and the amplitude of the initial transient
overvoltage. An accurate modelling of the elements of network and overvoltage
source parameters permits by mean of numerical simulations the determination of
the transmitted overvoltages [1]. In order to highlight the influence factors to the
overvoltage received by the final low voltage user, an existing network was
modelled and different transient events were simulated in some points. The
simulations were performed using EMTP – ATP software package.
2. The main features of the network and the modeling of their
elements
Fig. 1 presents the studied network which include a 110 kV overhead line
connected to the national system, a high voltage/medium voltage (HV/MV) step
down transformer which supply the 20 kV busbar of a substation, and many
overhead distribution lines, one of them supplying two transformer substations
namely TS1 and TS2. The length of high voltage line is 18 km.
Each transformer substation contains a single 20/0.4 kV, 250 kVA power
transformer with Dyn winding connections; the first transformer substation is
connected close to the 20 kV busbar by a cable line having 70 m length, and the
second by mean of a 31 km overhead distribution line. The medium voltage
terminals of transformers are protected by gapless arresters with 10 kA rated
current and 81.6 kV residual voltage. Local low voltage (LV) networks,
represented by 100 m overhead lines (with 35 mm2 cross section of Al conductors)
are supplied by TS1 and TS2 respectively. Two cases were considered: first (the
worst case) with no load in the secondary of MV/LV transformers and the second
one with a load of 40 Ω connected at the end of the low voltage line.
All power lines, except those of local low voltage were modelled by their
distributed parameters, the input data being rigorously known. For the step-down
transformers it was adopted a three leg core saturable power transformer model
completed with a capacitor network. The capacitances in the equivalent circuit of
transformers were provided by low frequency measurements during maintenance
operations.
An adequate metal oxide arrester model was introduced in the equivalent
diagram of the network. The ground of transformer substations was considered by
mean of a ( Rg = 4 Ω) invariable resistance.
In order to evaluate the parameters of overvoltages transmitted to the
inferior voltage networks, overvoltages with different shape and amplitude were
applied in different locations.
Influence factors on the transmitted overvoltages from high voltage to low voltage networks 185
3. The response of the network to the different overvoltages
The events which produce the transients in the high and medium voltage
network were considered as follow: full and chopped wave lightning overvoltages
and lightning induced overvoltages (with short tail). The strokes were considered
to be located both on the 110 kV overhead line and on 20 kV medium voltage
network.
In the first set of simulations, a full lightning impulse voltage having
standardized testing shape (1.2/50 µs) and 600 kV amplitude was injected at the
beginning of 110 kV overhead line, on the A phase. The overvoltages which act at
the high voltage terminals of 110/20 kV transformer are presented in Fig.2.
Equivalent
source 110 kV
Equivalent load of 20 kV busbar
HV-OHL - 18 km
CL- 70 m
Transformer
substation no.1 (TS1)
Transformer
substation no.2
(TS2)
OHL - 31 km
20 kV busbar
of 110/20kV
substation
Rg
Rg
Local LV
network
Local LV
network
Fig.1. The analyzed network.(HV-OHL – 110 kV high voltage overhead line; MV-OHL –
20 kV medium voltage overhead line; CL – cable line; TS – transformer substation)
It can observe that on the phase A, the maximum recorded peak value of
the voltage does not change substantially comparing with amplitude surge injected
at the beginning of the line (in the first stage, due to propagation along the line,
the amplitude of voltage arrived at the terminals of high voltage transformer is
reduced to approx. 525 kV but the reflexion that occurs restore it, practically, to
their initial value after about 30 µs). On the phases that were not directly affected
(B and C), identical induced voltages occur reaching peak values about 50% of
phase A. An oscillatory regime in the primary windings of transformer is
triggered. The voltage occurring at the medium voltage terminals present a slow
time variation with an amplitude about 50 times lower compared with those of the
primary one, the reduction being with one order of magnitude greater than rated
transformer ratio (which is 5.5) (Fig. 3).
186
Marian Costea, Bogdan Nicoară
700
700
[kV]
[kV]
525
2
525
phase A
350
350
175
175
0
0
-175
-175
phases B & C
-350
-350
-525
1
-525
-700
0.04
0.06
0.08
(f ile LOVDIR1.pl4; x-v ar t) v :T11R
0.10
v :T11S
0.12
0.14
0.16
0.18 [ms] 0.20
v :T11T
-700
0.0
0.3
(f ile LOVDIR1.pl4; x-v ar t) v :T11R
factors:
1
1
offsets:
0
0
Fig.2. The line-to-ground voltages at high voltage
terminals of 110/20 kV transformer, after the
propagation of the surge along 110 kV line.
0.6
0.9
1.2
[ms]
1.5
v :AT1R
50
0
Fig.3. The line-to-ground voltages at primary
(phase A – 1) and secondary (phase A’ – 2; these
values are multiplied by 50) terminals of
110/20 kV transformer due to the lightning stroke.
An expanded time diagram of the voltage variations between MV
terminals and earth of the two MV/LV transformers in the transformer substations
TS1 and TS2 is presented in Fig. 4. The oscillatory regime has a basic frequency
of about 33 Hz and amplitude of 12 kV. It can be remarked that the overhead
distribution line does not modify, practically, the shape and the amplitude of the
transmitted overvoltage arrived to the far away transformer substation (TS2). As a
result of slow variation of the input overvoltage, in the low voltage network a
reduced value is injected, according to the transformer ratio (see Fig.5), and the
final user is not affected (the maximum peak voltage reaches only 240 V on the
phase C).
300
12
TS1–phase A
[kV]
phase C
[V]
8
200
4
100
0
0
-4
-100
-200
-8
TS2–phase A
phases A &B
-300
-12
0
2
(f ile LOVDIR1.pl4; x-v ar t) v :BA1R
4
6
8
[ms]
10
v :BAR
Fig.4. The line-to-ground voltages recorded at
medium voltage terminals of the two transformers
(TS1 and TS2).
0
2
(f ile LOVDIR1.pl4; x-v ar t) v :T52R
4
v :T52S
6
8
[ms]
10
v :T52T
Fig.5. The line-to-ground voltages recorded at
low voltage terminals of TS2 transformer.
Another set of simulations was performed using a tail chopped lightning
overvoltage injected in the high voltage line, on the phase A and having also the
amplitude of 600 kV. The voltages in the primary of 110/20 kV transformer are
presented in Fig. 6, the maximum peak being recorded on the phase A, which was
directly affected by the lightning. Despite the rapid time variation of the voltage
across primary winding of transformer, in this case a very important decrease of
Influence factors on the transmitted overvoltages from high voltage to low voltage networks 187
overvoltages transmitted to the medium voltage network was observed and the
voltage reaches only 2 kV on MV busbars. It can conclude, taking into account
even the network modelling imperfection that a lightning stroke, full or chopped,
injected in the 110 kV line, have practically no effect to the final low voltage
users. Other simulations were performed, considering that the lightning acts
directly or indirectly on the 20 kV distribution network.
150
700
[kV]
phase A
[kV]
TS1
125
500
100
300
TS2
75
100
50
25
-100
0
phases B & C
-300
-25
-500
0.00
0.05
(f ile LOVDIR2.pl4; x-v ar t) v :T11R
0.10
v :T11S
v :T11T
0.15
0.20
[ms] 0.25 -50
0.00
0.05
0.10
(f ile LOVDIR4.pl4; x-v ar t) v :BAR
0.15
0.20
0.25
0.30
0.35 [ms] 0.40
v :BA1R
Fig.6. The line-to-ground voltages at high
Fig.7. The line-to-ground primary voltages at
voltage terminals of 110/20 kV transformer, 20/0.4 kV terminals of transformers connected in
after the propagation of the chopped lightning TS1 and TS2 (for the last, the delay due to the
overvoltages along 110 kV line.
propagation can be observed).
In the first case, a full lightning overvoltage having 165 kV peak value is
injected in the MV busbar of substation. As consequence, an important
overvoltage occurs in the low voltage network. At the medium voltage terminals
the peak voltage reaches about 145 kV for TS1 – the closer transformer to MV
busbars – and about 60 kV for TS2 – the remote transformer (Fig. 7). Rate
attenuation due to the medium voltage line of about 2.7 kV/km can be remarked.
At the low voltage terminals of these transformers, the amplitude of overvoltages
reaches about half of the primary voltages (transmission ratio 2:1), a very serious
stress for the low voltage insulation and the final user, even if their duration is
short. This stress duration is about 50 µs for TS1 and about 20 µs for TS2. The
necessity to install voltage arresters to LV terminals of transformer is
consequently proved.
The other simulations, with increasing amplitude of overvoltages
occurring on MV busbars, reveal that the voltages recorded at secondary terminals
do not increase directly proportional with the primary ones because of surge
arrester intervention. Keeping the amplitude of lightning and the affected phase
(A) but changing their shape (at 1/5 µs) the amplitude of overvoltage in the
secondary windings of transformers located in TS1 and TS2 remains at a high
level (for example, about 50 kV for TS1) but the stress duration become under
10 µs. Also reducing the coupling capacitances between MV and LV windings
(from 5 nF to 1 nF) of transformers has a reduced influence to the peak and time
duration of transmitted overvoltage.
In the last series of simulations, lightning induced overvoltages (LIOV)
188
Marian Costea, Bogdan Nicoară
were considered to be injected in the medium voltage network.
The coupling mechanism between the lightning channel and a medium
voltage overhead line is not discussed in this paper. It has been treated in
numerous works such as [2...5]. The main conclusions which can be found about
the features of this type of overvoltage are the following:
ƒ the amplitudes reached by the lightning induced overvoltages (generally,
under 300 kV for medium voltage overhead lines) are dangerous only for the
distribution systems because their relatively reduced basic insulation level;
ƒ the shape of LIOV can be described such as short tail lightning impulse (the
front duration about 1 μs and the tail −few microseconds);
ƒ because of reduced distances between the phases of the medium voltage
overhead lines, the LIOV have identical amplitudes on all phases.
In our simulations a shape of 1/5 μs of the induced overvoltages was
considered with the amplitude equal to basic line insulation (125 kV), all three
phases being equally stressed. The point of overvoltage injection was considered
the 20 kV busbar of substation. While the medium voltage terminals of the
transformers are protected by surge arresters, no low voltage arresters at
secondary terminals of 20/0.4 kV transformer were considered in the first
simulation.
The notations used to describe the results are: U1 − the amplitude of the
voltages between the medium voltage terminals and ground of the transformer
substation; U10 − the amplitude of the voltages between the medium voltage
terminals and reference ground; U2 and U20 the amplitude of the voltages between
the low voltage terminals and ground and, respectively, reference ground. The
main results of this simulation are presented in the Table I, for the case when no
load connected at low voltage consumer.
Table 1
The amplitudes of transient voltages without arresters at low voltage terminals of
transformer substations
Transformer
U1
U10
U2
U20
substation
kV
TS1
72.0
111.0
14.1
34.9
TS2
51.0
52.8
4.1
5.6
The differential voltage peak at TS1, between a phase conductor and the
neutral (connected to the common ground of the substation) reaches about 14 kV,
with a rise time shorter than 1 μs and the total duration of transients about 10 μs.
The transient ground potential rise (above 30 kV) is transmitted to the neutral
conductor of low voltage system. It must be noted that in the transformer
substations in Romania the usual case is that a single ground exist for medium and
low voltage parts. Regarding TS2, located far away from the MV busbar, the
amplitudes of the overvoltage at the primary terminals of transformer are reduced
about 2.1 times comparing to the similar stress of TS1 (the rate attenuation due to
Influence factors on the transmitted overvoltages from high voltage to low voltage networks 189
the medium voltage line is now about 1.9 kV/km, lower than the corresponding
value for the shape 1.2/50 µs, as seen previously), the amplitude of the phase to
ground voltage being limited at 4 kV. The duration of the transient process has the
same order of magnitude as for TS1.
For the TS1 the ratio between the primary and the secondary amplitude of
the overvoltages is about 5.1, while for TS2 this ratio increases up to 12.4. Greater
ratio means a diminished coupling between the primary and the secondary
windings of the transformer. No association with the rated transformer ratio (50,
for power frequency voltage) can be found in the case of fast transient voltage
applied to the primary windings. The average steepness of the overvoltage
decreases by propagation along overhead line from 95 kV/µs at medium terminals
of TS1 to about 17 kV/µs at TS2 equivalent terminals. In order to avoid great
disturbances or insulation faults in the low voltage network supplied by 20/0.4 kV
transformer, LV arresters must be connected at their secondary terminals.
For subsequent simulations low voltage arresters (having clamping voltage
Up=1.8 kV and rated current 10 kA, 8/20 µs) were connected to low voltage
terminals of transformer. Even in the case of the most exposed equipment (TS1),
the secondary voltages, for the same dielectric stress is reduced up to 1.3 kV (and
about 1 kV for TS2).
The presence of LV arresters has no influence, of course, to the potential
difference between secondary terminals and reference ground which remain very
high for the transformer closest to the origin of lightning induced overvoltage.
Then, adequate measures must be taken to low voltage consumer (such as the
achievement of own ground and the use of surge protection devices in the point of
coupling to the network).
4. Conclusions
During the lightning storms numerous disturbances in the low voltage
networks occur, even if this network in not directly affected, i.e. the lightning
activity is far away from the network. The paper analyze how a lightning
overvoltage occurred in a superior voltage level (high or medium) could be
transmitted to the inferior voltage network.
Different shape of overvoltages (defined as full impulses 1.2/50 µs or 1/5
µs and 1.2/50 µs chopped impulses) were injected in a model of real network
existing in Romania, and the response of power lines and transformers which
connect the networks with different rated voltage was analyzed. The main interest
was to record the disturbance level to the low voltage final user.
For the case of overvoltages assumed to have a full standardized lightning
impulse shape or tail chopped impulse occurred in a high voltage overhead line
(rated voltage 110 kV), the transmitted transients to the medium voltage network
190
Marian Costea, Bogdan Nicoară
through step down transformer show no significant amplitudes. On a relatively
short line (having 18 km) the attenuation due to the propagation is reduced. Also
the chopped impulse was stronger attenuated than the full one. At the same
amplitude of lightning overvoltages (full or chopped) the behavior of transformer
is different, the greater amplitude characterizing the full impulse.
The final low voltage consumer can be affected only if the overvoltage is initiates
in the medium voltage network. In this case great values of amplitudes at secondary
terminals were recorded both for a full impulse 1.2/50 µs acting on single phase and for
full impulse 1/5 µs acting to all three phases. The greatest values were obtained for a
single phase insulation stress. But the probability of occurrence of this last kind of
overvoltages (due to a direct lightning stroke) is reduced.
More probable for MV overhead line is the occurrence of induced lightning
overvoltages acting to all three phases. An interesting result is the identically response of
MV/LV transformer at primary terminals single phase excitation comparing with all
phase excitation, but only for the transformers located in the vicinity of injection point of
overvoltage. For this reason, the reduced values were followed to the final user in order to
conclude about disturbance level and protective measures to be adopted.
The propagation along the MV distribution line reduces the steepness of the
overvoltage more for the shape of 1.2/50 µs compared with the shape of 1/5 µs. The most
exposed low voltage network is that supplied by the nearest transformer to the striking
point, particularly in the case when no load is connected. The simple existence of a
ground at the low voltage consumer does not solve the problem of overvoltage
transmitted to it even if adequate low voltage arresters are connected in the transformer
substation. And this is because of the resistive coupling path between the ground of the
substation and the ground of the consumer by the neutral conductor which transmits the
potential ground rise. A combined set of measures (own ground and surge protection
device at the connection point) must be adopted in order to protect the low voltage
consumer and reduce their insulation stress to the level prescribed by representative
standards such as [6].
REFERENCES
[1] * * * – CIGRE WG 02 SC 33 – Guidelines for representation of network elements when
calculating transients Brochure CIGRE no. 39, 2000.
[2] C.A. Nucci, F. Rachidi – Lightning-Induced Overvoltages, presented at the IEEE Transmission
and Distribution Conference, Panel Session "Distribution Lightning Protection", New
Orleans, USA, April 14, 1999.
[3] V. Cooray, V. Scouka – Lightning-Induced Overvoltages in Power Lines: Validity of Various
Approximations, IEEE Trans. on Electromagnetic Compatibility, vol. 40, no. 4, pp. 355363, Nov.1998.
[4] C.A. Nucci, F. Rachidi, M.V. Ianoz, C. Mazzetti – Lightning-induced voltages on overhead
lines, IEEE Trans. on Electromagnetic Compatibility, vol. 35, no. 4, pp. 75-86, Feb.1993.
[5] A. Borghetti, F. Napolitano, C.A. Nucci, M. Paolone, M., A.S. Morched – Lightninginduced overvoltages transferred from medium-voltage to low-voltage networks, Proc.
PowerTech 2005, St. Petersburg, Russia, 27-30 June 2005, pp.1-7.
[6] Insulation coordination for equipment within low voltage systems. Part 1: Principles,
Requirement and Tests, IEC 60664-1/2000.